This invention relates to optical null position sensors and more particularly to a compensation scheme for substantially eliminating changes in optical output efficiency due to variations in temperature.
One class of accelerometer employs a pendulous mass disposed between a light emitting source such as a light emitting diode and photodiodes responsive to light. In particular, the photodiode may be a monolithic device having two elements, each of which is responsive to light radiation. Under acceleration, the pendulous mass will deflect so that one of the photodiode detectors will receive more illumination than the other. The relative amounts of illumination can be measured and differenced to generate an error signal which can be used to drive the pendulous mass back to a null position. Oftentimes the pendulous mass is driven back to null by means of electric coils on the mass interacting with a permanent magnet. The amount of current in the coils required to maintain the pendulous mass at null is then an indication of the acceleration of the accelerometer and the vehicle to which it is attached.
Such a system is a closed loop one in that deviations from null are measured and fed back to reestablish the null condition. As in all closed-loop systems, gain levels must remain within certain limits or else the system can go unstable. Thus, if the overall gain of the light emitting diode/photodetector system were to change, the corresponding gain might be such as to destabilize the system. Changes in gain can come about as a result of component variations as a function of temperature.
One way to compensate for changes in temperature would be to measure temperature directly and use this reading to compensate the readings from the photo-detectors. This technique, of course, requires separate temperature sensing elements which add to the cost and complexity of the accelerometer apparatus.
It is therefore an object of this invention to provide an accelerometer having an optical sensing system and incorporating temperature compensation without the need for measuring temperature directly.
It is a further object of the invention to provide such a system which is highly effective and reliable.
A still further object of the invention is such a system which is simple and inexpensive to build and operate.